The process of formation of blood cells is called haemopoiesis and the tissues which form blood corpuscles are termed the heamopoietic issues. In frog, spleen, liver and lymph nodes are main sites of haemopoiesis. During rains haemopoiesis also occurs in bone marrow due to an active life of frog. In tadpoles kidneys are also haemopoietic.
In mammals, yolk sac live, bone marrow, lymph nodes, spleen and thymus are the haemopoietic organs in the embryo. In adults most of the blood corpuscles are formed in the red bone marrow of long bones-